The present invention relates to a thermosensitive recording material which is, in particular, improved with respect to the stability of recorded images. The thermosensitive recording material comprises a support material and a thermosensitive color-forming layer formed on the support material, which thermosensitive color-forming layer comprises a leuco dye which is a colorless or light-colored at room temperature, the antipyrine complex of zinc thiocyanate (hereinafter referred to as the zinc thiocyanate antipyrine complex) which serves as a color developer capable of inducing color formation in the leuco dye upon application of heat thereto. When necessary, other auxiliary sensitizer agents can be used in combination with the zinc thiocyanate antipyrine complex.
Recently thermosensitive recording materials are employed in a variety of fields, for instance, for use with printers of computers, recorders of medical analytical instruments, facsimile apparatus, automatic ticket vending apparatus, and thermosensitive copying apparatus, since they have the following advantages over other conventional recording materials: (1) Images can be formed by simple heat application, without complicated steps for development; (2) the thermosensitive recording materials can be produced by a simple apparatus and the storage of the thermosensitive recording materials is simple and does not involve excessive costs; (3) as the support material of the thermosensitive recording materials, paper is usually used, which is rather inexpensive in comparison with other support materials, such as synthetic resin films, and when paper is used as the support material, the thermosensitive recording material has a pleasing plain-paper-like touch.
As such a thermosensitive recording material, there is proposed, for instance, in Japanese Patent Publications Nos. 43-4160 and 45-14039, a thermosensitive recording material which comprises a support material and a thermosensitive coloring layer containing a colorless or light-colored leuco dye including a lactone, lactam or spiropyran ring, and an acidic material such as an organic acid and a phenolic material which serves as a color developer capable of inducing color formation in the leuco dye upon application of heat thereto. This thermosensitive recording material, however, has the shortcoming that the recorded images become easily discolored or fade away when the recorded images come into contact with oils and a plasticizer, such as dioctyl phthalate, contained in plastic films.
Conventionally, a variety of thermosensitive coloring layers are proposed which are improved so as to increase the stability of the recorded images. For example, a thermosensitive coloring layer containing a large amount of a resin which is resistant to water and chemicals, a thermosensitive coloring layer containing a resin which becomes hard upon exposure to heat or light, and a thermosensitive coloring layer having a protective layer made of a resin on the surface thereof are proposed. These thermosensitive coloring layers, however, have the shortcomings that the processes for preparing them are complicated, the photosensitivities are low and the costs are high.
Japanese Laid-open Patent Application No. 48-51716 and Japanese Patent Publication No. 51-25174 disclose a pressure-sensitive recording material containing a color developer which is resistant to chemicals. This color developer can be used with the pressure-sensitive recording material. However, when it is used in a thermosensitive recording material, it has the shortcoming that the background area is colored in contact with a plasticizer, although colored image areas are stable to a plasticizer.
Japanese Laid-open Patent Application No. 59-11286 with an invention title of "Pressure-sensitive or Thermosensitive Recording Material" discloses a number of color developers. When they are used in thermosensitive recording materials, most of them provide poor-quality thermosensitive recording materials, with conspicuous fogging in the background. Furthermore, the background of such thermosensitive recording materials is colored when it comes into contact with plasticizers.
In accordance with the recent general demand for high speed and highly condensed recording, there is not only a great demand for a high speed thermosensitive recording apparatus, but also a demand for a thermosensitive recording material that is sufficiently compatible with the high speed thermosensitive recording apparatus so as to be capable of attaining high speed and highly condensed recording, in particular, for use in the previously mentioned fields.
Conventionally, thermosensitive recording materials for use in high-speed recording have been proposed, for instance, in Japanese Laid-Open Patent Application No. 53-39139, Japanese Laid-Open Patent Application No. 53-26139, Japanese Laid-Open Patent Application No. 53-5636, and Japanese Laid-Open Patent Application No. 53-11036. In those thermosensitive recording materials, thermo-fusible materials having a low melting point, such as a variety of waxes, fatty acid amides, alkylated biphenyls, substituted biphenyl alkanes, coumarinic acid derivatives, biphenyl amines, are added to the thermosensitive coloring layer as a sensitizer or as an agent for reducing the melting point of the thermosensitive coloring layer. When the above described sensitizer agents are employed, it is necessary that the sensitizer agent be melted prior to the coloring reaction. Therefore, in the high-speed recording by application of a small amount of thermal energy in an extremely short time, for instance, by heat-application impulses, a sufficiently high thermal response for practical use cannot be obtained. In addition to the above problem, in the above case, since the thermo-fusible materials are melted within the thermosensitive coloring layer, the accumulation of the melted thermo-fusible materials on a thermal head during an image recording process, trailing of the printed images and formation of ghost images are apt to occur. Furthermore, at high temperatures and high humidities, fogging also occurs in the background of thermosensitive recording material during storage. As a result, the contrast of the recorded images decreases during storage.